A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
Typically, target portions on the substrate are separated by a scribe lane, in which one or more marks are located. For coarse optical alignment of a wafer in a lithographic apparatus, an optical alignment scan is performed along a scan path across the scribe lane over such a mark (thus scan data is also gathered outside the scribe lane). The optical alignment scan is based on so-called self-referencing interferometry to obtain a scan data signal from a mark. Such a mark typically comprises three parallel lines that extend in a longitudinal direction of the scribe lane and that are spaced apart from each other in a width direction of the scribe lane. A pitch between each pair of lines in the mark is different from a pitch between an other pair.
The used self-referencing interferometry has been described in EP 1372040. The position of the mark can be obtained by searching in the scan data signal for a signal portion that matches the two pitches of the mark-design, for example by a pattern recognition procedure. The location of the matching signal portion in the scan data signal relates to the position of the mark within the scan path.
It is observed that product structures, next to the mark and outside the scribe lane, may cause interference with the signal generated by the mark. The interference may cause inaccuracy in the pattern recognition and result in a misalignment.
Moreover, because any device structure (product structure) next to the mark will cause interference with the mark itself, this will disturb the alignment performance. Therefore, the mark has an exclusion zone which will be left empty without device structure. For that reason the scribe lane typically does not have a width below a minimal value. To reduce the width of the scribe lane may lead to a less accurate coarse alignment result and is therefore undesirable when using a coarse alignment method in accordance with the prior art.